Process for sizing in the production of paper, cardboard, paperboard and other cellulose containing materials

In a process for sizing in the production of paper, cardboard, paperboard and other cellulose-containing materials by natural sizing agents, the improvement comprising:

• sizing the cellulose-containing materials, under neutral to weakly basic pH conditions and without the use of aluminium salts, by the steps of first adding a natural sizing agent to a suspension of said materials and then adding an aqueous solution, containing up to 50% by weight of solids, of a condensation product consisting essentially of dicyandiamide with formaldehyde in molar ratio of from 1:1.0 to 1:4.0, said product having been prepared in the presence of at least one inorganic or organic acid and/or at least one ammonium or amine salt of an inorganic or organic acid.